Study refutes notion that diesel-powered
vehicles are better for the environmentEfforts to reduce air pollution should instead
focus on hybrid cars and low-emission gasoline-powered vehicles,
expert says

BY CZERNE M. REID

Diesel fuel is now at the center of a delicate balancing act
between smog production and global warming. Some lawmakers and car
manufacturers advocate widespread diesel use in passenger vehicles
as a strategy for reducing the production of so-called "greenhouse
gases" thought to cause global warming. But according to a new
study, replacing gasoline vehicles in the United States with diesel
vehicles -- equipped even with the most modern pollution controls
-- may increase smog production over most of the country.

"If provisions in last year's United States energy bill pass
this year, many diesel vehicles will qualify for the same tax
credits as hybrid and hydrogen fuel cell vehicles," said Mark
Jacobson, associate professor of civil and environmental
engineering and lead author of the study published January 30 in
the journal Geophysical Research Letters. He added, "It is a
surprise to me that this proposal was made before its possible
effect on public health was evaluated."

The main component of smog is ozone. In the upper atmosphere it
absorbs intense cancer-causing ultraviolet radiation from the sun
and prevents it from reaching Earth. In the lower atmosphere where
we breathe, however, ozone is a respiratory irritant associated
with decreases in lung function and increases in hospital visits
for respiratory causes. Children, the elderly and individuals with
preexisting respiratory disease have increased sensitivity to ozone
exposure.

Ozone is formed in the lower atmosphere through
sunlight-initiated chemical reactions between nitrogen oxides and
hydrocarbons -- compounds found in vehicle exhaust. The U.S.
Environmental Protection Agency has designated ozone and nitrogen
dioxide, along with four other air pollutants, as "criteria
pollutants" for setting concentration standards for the protection
of public health and the environment.

Compared to gasoline-fueled vehicles, diesel-fueled vehicles
emit more nitrogen oxides and hydrocarbons, as well as fine
particles linked to reduced lung and cardiovascular function. Last
year, less than 1 percent of all existing passenger vehicles in the
United States were fueled by diesel, but there is an increasing
trend toward diesel use. Supporters promote diesel vehicles as
obtaining 20 to 30 percent better mileage than do equivalent
gasoline vehicles. Such improved fuel efficiency should result in
lower emission of compounds that lead to the production of carbon
dioxide, a major greenhouse gas. But, according to Jacobson, diesel
does not provide a dramatic reduction in carbon emissions. Instead
only a modest 5 to 15 percent decrease results. This is because
diesel contains more carbon per gallon of fuel than does
gasoline.

"Modern" diesel vehicles do not, however, emit the black, sooty
exhaust characteristic of traditional diesel vehicles. This is
thanks to better engines, improved fuel mixes and enhanced
pollution control technologies such as particle traps and devices
for controlling nitrogen oxide emissions.

To examine the results of using cleaner diesel technology,
Jacobson and his colleagues at the California Institute of
Technology, University of Iowa and Argonne National Laboratory used
mathematical models to simulate the effects of replacing all
gasoline-fueled vehicles in the United States with "modern"
diesel-fueled vehicles. The scenario was "best case," since diesel
vehicles in the United States do not yet have pollution controls
such as traps and filters.

The researchers programmed data from the extensive U.S. National
Emission Inventory into their model. The inventory contains
emissions from hundreds of thousands of sources including 1,700
types of vehicle sources, of which 870 types are gasoline. The
researchers validated their model by comparing current conditions
with observations from the U.S. Ambient Air Quality database. The
computer model incorporated all the processes that affect pollution
in the atmosphere: emission, gas chemistry, particle processes,
meteorology, transport, radiation, clouds, and removal by rainfall
and deposition.

The main result of the study is that when gasoline vehicles were
switched with diesel, there was an increase in surface ozone over
75 percent of the United States, particularly in the Southeast.
There was a slight decrease over the remaining 25 percent, but on
average, surface level smog increased over the United States. In
the lower atmosphere, just above the surface, ozone levels
increased across the entire country.

In addition, the researchers found that pollutants other than
ozone also increased, and confirmed that to reduce ozone levels
over most of the United States, controlling vehicular nitrogen
oxide emissions would be more effective than controlling
hydrocarbon emissions. This is particularly true in the Southeast,
where natural vegetation emits high levels of hydrocarbons that
react with vehicular nitrogen oxide to form ozone.

Jacobson recommended low-emission gasoline-fueled vehicles and
high-mileage gasoline-electric hybrid vehicles as the best strategy
for reducing vehicle-related air pollution and climate problems
simultaneously.